Metal oxides may be formed by reactions of vapors of a wide variety of metal-containing compounds. These processes are called chemical vapor deposition (CVD) processes.
One widely-used family of metal CVD precursors are the metal alkoxides, having the general form M(OR).sub.n, in which M represents a metal atom, and R stands for an organic radical, such as methyl, ethyl, isopropyl, etc. Examples of this family include titanium isopropoxide, aluminum isopropoxide and tetraethylorthosilicate. When vapors of these alkoxides reach a heated surface, a layer containing the corresponding metal oxide is deposited on the surface.
The layers thus deposited, however, are usually found to contain significant amounts of carbon, resulting from incomplete removal of the carbon-containing organic radicals. This carbon contamination can result in deleterious effects on the properties of the metal oxide, such as increased absorption of light. One method for reducing this carbon contamination is to add an oxidant, such as oxygen or ozone, to the metal alkoxide vapor. Another sometimes beneficial effect of the oxidant is often to reduce the temperature required for the CVD process.
The presence of an oxidant vapor in the process may, however, also have deleterious effects. For example, in order to produce an electrically conductive doped titanium dioxide layer, it is advantageous to maintain a non-oxidizing atmosphere in the CVD process chamber. Addition of an oxidant gas into the CVD process results in an insulating doped titanium oxide film. In other cases, the oxidant may damage a substrate by oxidizing it or producing an undesirable oxide layer on the substrate. For example, silicon grows an insulating oxide layer on its surface when heated in an oxidizing atmosphere. If one wants to grow a metal oxide layer on the silicon without the intervening silicon oxide layer, then the CVD process must be carried out in the absence of an oxidant.
Another difficultly with metal alkoxide precursors arises when one tries to deposit mixtures of metal oxides. Each metal alkoxide has a narrow temperature range under which its CVD process operates. For example, titanium isopropoxide is usually used at temperatures from 400.degree. to 500.degree. C. whereas tetraethylorthosilicate does not decompose until the temperature is above about 600.degree. C., in the absence of an added oxidant.
Another class of metal oxide precursors are the metal halides, including fluorides, chlorides, bromides and iodides. Such compounds, lacking oxygen, obviously need an oxygen source to react with in order to produce a metal oxide film. Oxygen gas is sometimes used, leading in some cases to the difficulties discussed above for an oxidizing atmosphere. Water is sometimes used in a non-oxidizing atmosphere, but reaction with water vapor can be too rapid, leading to the formation of particulates which can contaminate the film and cause pin-holes and porous films.